Engine runs erratic

[Nitelord]

I have a 1984 Johnson 75hp that runs well very well..SOMETIMES! The engine will start right up and idle well. When at full throttle(sometimes) the engine will automaticly back down as if I had back off on the throttle. If I manully back off on the throttle when this occures it will "catch up" and run well. Today when I had it out on the water, the engine was running great, I had it at full throttle came into a no wake zone and manully throttle her down, after passing the no wake zone and throttle her back up, the engine would only go at about have throttle even though I had her at full throttle. I backed off on the throttle to idle (then it stalled but started right up) and tried it again with the same results. The engine still would run only at half throttle even though throttle was wide open. When I got her home out of the water the engine started right up and was able to throttle wide open, I tried to diplicate the problem but seems to happen only in the water. I can't figure out why it will run well for awhile and then poorly,back and forth...Any Help would be greatly appreciated.THX

 

[tashasdaddy]

Re: Engine runs erratic

DON NOT RUN ABOVE 1500 RPMS out of water. could be prime bulb collapsing. restricing fuel flow. vent not open on fuel tank, or blocked. could be coils or power pack failing. could be fuel pump failing.

 

[R.Johnson]

Re: Engine runs erratic

When an engine act's like this, it is generally loosing fire to cylinder. You could use a timing light to track down which cylinder is at fault, but don't rule out a fuel restriction. It is always best to start with a ignition, compression, fuel test, when working on any engine.This will save time, money, head scratching.

 

[wavrider]

Re: Engine runs erratic

Alot of different tests on fuel lines, vacuum test tels alot.

start with fuel delivery first, easiest and usually least expensive.

Look at vent on fuel tank, leaking or corroded fuel lines at connectors

Last time carbs rebuilt?

Check spark with timing light, with the LU in a barrel if possible so have back pressure, check all cylinders to see if you have spark.